class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
        next = null;
    }
}


public class DetectCycle {
    public boolean detectCycle(ListNode head) {
        ListNode fast=head;
        ListNode slow=head;
        while(fast!=null&&fast.next!=null){
            slow=slow.next;
            fast=fast.next.next;
            if(fast==slow){
                return true;
            }
        }
        return false;
    }
}